The present invention relates to a method and apparatus for detecting a fetal heart rate by autocorrelating an ultrasonic Doppler signal reflected from a fetal heart and then detecting the fetal heart rate based on the periodicity of the autocorrelation curve.
To monitor the well-being of a fetus in the process of the delivery, it is highly effective to simultaneously and continuously record a fetal heart rate curve and a labor curve. One known process of measuring the fetal heart rate (hereinafter referred to as "FHR") is composed of the steps of emitting an ultrasonic wave toward a fetal heart and utilizing a reflected ultrasonic Doppler signal which has been subjected to a frequency shift. However, since such an ultrasonic Doppler signal representative of one heart beat is quite complex due to complicated beating of the heart, it has been difficult to detect a train of pulses which are precisely synchronous with the corresponding fetal heart beats.
U.S. Pat. No. 3,982,528 discloses an apparatus for detecting the FHR by autocorrelation technique to eliminate the above problem. FIG. 1 of the accompanying drawings is a block diagram of the disclosed apparatus. Included in the apparatus is an ultrasonic transducer 1 and a driver 2 for the ultrasonic transducer 1. A pre-processor 3 is provided and includes an amplifier, AM detector, and a filter for detecting a frequency-shifted signal from a received signal from the transducer 1. An audio output means 4 produces an audio output indicative of a fetal heartbeat signal f(t) issued from the pre-processor 3, and an autocorrelator 5 autocorrelates the output signal f(t) from the pre-processor 3. A post-processor 6 detects the periodicity of a pattern of autocorrelation function signal .phi.(.tau.) generated by the autocorrelator 5. A pulse-interval-to-heart-rate converter 7 issues a step-like signal by calculating a heart rate 60/T (heartbeats/minute) from a time interval T sec. of a train of pulses generated by the post-processor 6.
The post-processor 6 comprises a peak detector 61 for detecting a new peak by comparing the level of the signal .phi.(.tau.) and a level related to its immediately prior peak, and a delay means 62 for issuing a peak signal only when there is no next peak detected within a predetermined time after the peak has been detected by the peak detector 61. The autocorrelator 5 supplied with the output signal f(t) (FIG. 2A) from the pre-processor 3 autocorrelates the supplied signal in a digital manner, converts the processed signal into an analog signal, and then issues the analog signal .phi.(.tau.) (indicated by the solid line in FIG. 2B).
The peak detector 61 compares the immediately prior peak reference level (indicated by the dotted line in FIG. 2B) with the signal .phi.(.tau.) to detect the peak (FIG. 2C). The delay means 62 issues the pulse signal (indicated by the solid line in FIG. 2D) to the converter 7 when no next peak is detected in the predetermined time after the peak has been detected. The converter 7 issues the step-like FHR signal (indicated in FIG. 2E on a compressed time base) of the level corresponding to the interval of the pulse signal.
As shown in FIG. 2A, the fetal heartbeat signal obtained by the ultrasonic Doppler process frequently contains a few waveforms synchronous with heartbeats per heartbeat. This is because movements of the heart valves, heart wall, blood flow, and others are detected as signals when the heart is beating. When the autocorrelation function is calculated under such condition, there appear false peaks other than the true peak used for detecting a heart rate, resulting in a tendency to cause an error in detecting the true peak. Such an error is also caused by signal noise. More specifically, as shown in FIG. 2B, the signal .phi.(.tau.) usually contains false peaks b, c before and after the true peak (hereinafter referred to as a "main max.").
When the peak detector 61 detects the main max. a and the peak b before the main max. a, the delay means 62 issues only a proper peak signal indicative of the main max. a. However, if the peak b is of a level exceeding the main max. a, then the main max. a does not necessarily reach the peak reference level, and cannot be detected. If the detected level of the peak c is high, then the delay means 62 responds to the peak c and issues a false peak signal which is delayed from the main max. a. Furthermore, it is impossible to discriminate the main max. from high-level noise. Therefore, the known apparatus for detecting FHR signals has had limited accuracy.